The present invention is a fibrous, cationic cellulose pulp product and the method for preparing it. The product is especially advantageous in papermaking for its improved retention of certain dyestuffs and filler materials.
The surface of cellulose fibers is normally slightly anionic in nature due to the presence of carboxyl and carbonyl groups introduced during the pulping and bleaching process. This negative charge is responsible for a number of undesirable effects in papermaking. Principal among these is the tendency of the longer fibers to repel fine cellulose particles which result from refining and the similar tendency to repel many additives such as fillers, pigments, dyes, and sizes, many of which also bear negative charges. As a result, these fine particles tend to go into the white water during sheeting where they represent an economic loss and a pollution problem. In response to this problem, alum has traditionally been added to adjust the electrical charge of surfaces to which it is adsorbed. However, alum is not very efficient; therefore, relatively large amounts are required. This produces an undesirable, relatively highly acidic environment both in the sheeting process and in the final paper product. In the papermaking process, this acidity tends to corrode equipment. In paper it results in relatively rapid loss of physical properties such as tear strength and fold resistance.
A number of routes have been explored using materials besides alum to overcome the anionic nature of cellulose fibers. One such route, which has seen commercial use for approximately 30 years, has been the use of additives which are cationic in nature; e.g., cationic starch. These additives are attracted to the anionic cellulose and serve to modify or neutralize the electrical charge so that the fibers have less tendency to repel anionic additives. Today a relatively wide variety of cationic papermaking additives are available. These include materials for improving drainage rate, reducing fines and pigment loss, and increasing wet strength. Cationic additives also make the use of less acidic sizing agents possible. Alkyl ketene dimers are such a sizing agent applied in the pH range of 6-8. Articles to McKenzie, Appita 21(4): 104-116 (1968) and to Moore, Tappi 58:99-101 (1975) are informative of the state of the art.
Another route to overcoming the anionic nature of cellulose fibers has received considerable research although no products have yet evolved which have been of commercial importance. This approach has been to make the fibers themselves cationic in nature, usually by reaction with a material that introduces positively charged nitrogen atoms into a substituent side chain. Uwatoko, Kagaku Kogyo (Japan) 25 (3):360-362 (1974) briefly summarizes the state of the art in regard to cationic fibers. Uwatoko lists six major approaches that have been taken. Without putting them in any chronological order, these are as follows: the first method introduces side chains containing a tertiary nitrogen atom. These side chains are attached to the cellulose molecule at the hydroxyl groups as ethers. One product of this type which has received considerable study is the quaternized diethylaminoethyl derivative of cellulose. A second route to the preparation of cationic cellulose is the reaction of cellulose in the presence of sodium hydroxide with ethanolamine, aqueous ammonia or melamine. A third process is the reaction between cellulose and a material such as 2-aminoethyl sulfuric acid in the presence of sodium hydroxide. Another product has been formed by iminating an aminated cellulose by reaction between the aminated cellulose and ethylene imine. An approach which has received considerable study is the reaction of various trimethyl ammonium salts. Of particular importance has been glycidyl trimethyl ammonium chloride reacted with cellulose in the presence of a catalytic amount of sodium hydroxide. A related approach has been the reaction of 2-chloroethyldiethyl amine with alkali cellulose. This product is then quaternized with methyl iodide in anhydrous alcohol. Finally, Uwatoko describes a modified cellulose described in more detail in J. Soc. Fiber Sci. Technol. (Japan) 30 (5/6):T313-314 (1974). In this process cellulose is reacted with a solution of sodium acid cyanamid at a concentration of 50-200 g/L at a pH in the range of 10-13 and temperature of 10.degree.-40.degree. C. for 4-24 hours.
One approach not specifically discussed by Uwatoko is the reaction of cellulose with a mixture of epichlorohydrin and a tertiary amine with cellulose in the presence of aqueous sodium hydroxide. This process is discussed by McKelvey and Benerito in J. Appl. Polymer Sci. 11:1693-1701 (1967). Paschall, in U.S. Pat. No. 2,876,217 describes the use of this process to make a granular cationic starch useful as a papermaking additive. Benerito et al., Anal. Chem. 37:1693-1699 (1965) describe in detail the production of quaternary ammonium ethers of cellulose by the reaction of diethylaminoethyl cellulose with either methyl iodide or ethyl bromide under completely anhydrous conditions.
Kaufer et al., Papier (Darmstadt) 34(12):575-579 (1980) describes several applications of cellulose made cationic by the reaction of glycidyl trimethyl ammonium salts. These authors also teach the usefulness of .beta.-methacryloxyethyltrimethyl ammonium chloride as a cationizing agent.
Krause et al., Papier (Darmstadt) 35(IOA):33-38 (1981) building on the work of Kaufer and his coworkers, show the superiority of cationic pulps in retaining alkyl ketene dimer sizing materials as opposed to the conventional use of cationic starches as retention aids.
In West German Pat. No. 2,817,262, John et al. show that only part of the fiber in a papermaking stock needs to be cationized in order to achieve significant benefit.
Stone et al., in Canadian Pat. No. 838,684 teach the preparation of a wide variety of quaternary nitrogen-containing cellulose ethers which function as cationic materials.
Lewis et al., in U.S. Pat. No. 3,694,393 show the treatment of cellulose with the reaction product of epichlorohydrin and dimethylaminoethyl methacrylate.
There appear to be a number of reasons why a cellulose pulp having cationic substituents has never appeared commercially in the marketplace as a papermaking fiber. One of the principal reasons is the expense. In many cases the raw materials themselves are very expensive. Along with this is the problem that the reaction conditions of the cellulose with the substituent materials are such as to cause the cost of the product to be greatly elevated. Many of the cationic cellulose materials produced by straightforward chemical reaction are not of fibrous nature. This is a problem with relegates them to the nature of an additive in papermaking as opposed to use as a primary fiber. A number of the products which are fibrous must be produced by grafting reactions. Here free radical sites are induced in the cellulose chains by means such as ceric ion activation or high energy irradiation. An appropriate polymerizable monomer having vinyl unsaturation is then coupled to the cellulose and polymerized in the presence of a free radical initiator. The overall result has been a group of products which are either technically unsuitable or far too expensive for general use.
Cationic starches, which have been available commercially for over 30 years, do have some relationship to the cationic celluloses just described. One who sits on the edge of this particular scientific art might question why the processes used for the preparation of cationic starches have not successfully been applied to cellulose fibers. There is a ready answer. In the first place, most of the cationic starches are modified in physical nature by cooking or partially cooking during the chemical reaction which introduces cationic sites. There is not any need for these products to retain their original physical form. A second reason is that cationic starches are used in relatively small percentages in papermaking. Therefore, they form only a small portion of the ultimate product. This fact makes their relatively high costs more tolerable to the papermaker. While there is no need to review all of the extensive technical literature relating to cationic starches, a few recent patents bear some relationship to the present invention. Aitken, U.S. Pat. No. 3,674,725 describes a product in which a polyepichlorohydrin is modified with an amine, preferably trimethylamine. This product can then be reacted with a starch under strongly alkaline conditions. The same inventor, in U.S. Pat. Nos. 3,854,970 and 3,930,877 teaches an approximately equal molar composition of epichlorohydrin and dimethylamine reacted under alkaline conditions and then acidified to produce a quaternary ammonium salt. The preferred compositions have 10-20% ammonia substituted for an equivalent of the dimethylamine. These condensates can be used to prepare liquid cationic starches by reaction under rather strongly alkaline conditions with partially hydrolyzed starches. Buikema, U.S. Pat. No. 4,029,885, shows the use of those starches for sizing paper. Buikema et al., U.S. Pat. No. 4,146,515 treat a lightly oxidized starch with an epichlorohydrin-dimethylamine condensate at about 60.degree.-80.degree. C. for one hour. This product is subsequently acidified to make an amine salt. Cosper et al., U.S. Pat. No. 4,268,532, use a dimethylamine-epichlorohydrin polymer with a second polymer (which may or may not be anionic) for retaining starch in repulped broke. It is interesting that these inventors do not appear to have considered the possibility of reacting their epichlorohydrin-dimethylamine condensate with cellulose to produce a product which could be both fibrous and cationic in nature.
The present invention describes a cationic cellulose made by reaction, under mildly alkaline aqueous conditions, of cellulose fibers with one of a group of condensates based on the reaction product of epichlorohydrin and dimethylamine. The reaction conditions and nature of the materials involved is such that a fibrous product results which a little more expensive to manufacture than the cellulose itself.